CODEWITHSUNDEEP
clojure
tobiasbayer
tobiasbayer

Reputation: 10389

Wrong number of arguments for a simple function

Can someone explain the behavior in the Clojure code below? I don't get it. Does Clojure somehow replace or "optimize" function arguments? Why does calling a function with a single nil argument result in an ArityException?

(defn foo [bar] (reduce #(%1) bar))

(foo nil)
-> ArityException Wrong number of args (0) passed to: test$foo$fn  clojure.lang.AFn.throwArity (AFn.java:437)

Upvotes: 1

Views: 695

Answers (1)

Michał Marczyk
Michał Marczyk

Reputation: 84369

See (doc reduce):

[...] If coll contains no items, f must accept no arguments as well, and reduce returns the result of calling f with no arguments. [...]

Here coll is nil, which is effectively being treated as a collection containing no items (as it usually is in similar contexts), and f is #(%1).

Thus #(%1) is being called with no arguments and ends up throwing the exception you see.

Upvotes: 5

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